As a method of forming a high-quality thick semiconductor film, there is an epitaxial growth technique for growing a single crystal film on a substrate such as a wafer by vapor phase growth. In a vapor phase growth apparatus using the epitaxial growth technique, a wafer is placed on a support unit in a reaction chamber held at a normal pressure or a reduced pressure. While the wafer is heated, a process gas such as a source gas which is a raw material used in the film-formation is supplied from, for example, a shower plate disposed in an upper portion of the reaction chamber to a surface of the wafer. A thermal reaction of the source gas occurs on the wafer surface, and an epitaxial single crystal film is formed on the wafer surface. Recently, as a material of a light emitting device and a power device, a GaN (gallium nitride)-based semiconductor device has been attracted attention. As an example of an epitaxial growth technique for film-forming a GaN-based semiconductor, an organic metal vapor phase growth method (MOCVD method) is used. In the organic metal vapor phase growth method, organic metal such as trimethylgallium (TMG), trimethylindium (TMI) and trimethylaluminum (TMA), ammonia (NH3), or the like is used as the source gas. Hydrogen (H2) or the like may be used as a separation gas for suppressing a reaction between the source gases.
In the epitaxial growth technique and particularly in the MOCVD method, in order to form a uniform film on a wafer surface, it is important to suitably mix the source gas, the separation gas, and so on to supply the mixed gas in a uniform rectified state to the wafer surface. JP-A 2001-81569 describes a method in which in order to suitably mix different gases, source gases are separated into different gas diffusion chambers until the source gases are introduced into a reaction chamber.